If there's a battery compartment, you could do a resistance measurement between the DC connector's contacts, and the most positive / most negative contacts in the battery compartment. Results usually make it clear what is + and - for the DC connector.
Otherwise, if manual doesn't say, AND you don't have original DC adapter that comes with the recorder, AND recorder itself has no relevant markings, AND there's no battery compartment (you unlucky f... :x ), only way would be to open up the recorder & check out the circuit inside.
(or just gamble but risk damaging the recorder - tip positive is the most common arrangement but there are many exceptions)
What brand & model cassette recorder are we talking about here?